just gunna throw my 2 cents in just because i feel like its getting way to system crazy in here.
I think its way less complicated then suggestions are staring to go.
Sure, you can replumb your house, go to the local water supply, testing their supply lines and export lines, extract helium 3 from the moon using it to refine the purest base water you can create while balancing your tank on the back of a chair at an exact 37 degree angle with a hot pad in 1 hand as the other paddles the water precisely every 28 seconds, and blah blah blah.
But i think that all, while not bad, are a waste of your time.
I thought most your water params were close enough to acceptable and your equipment sounds great. Cal, n03, and KH all sounded a little lower then i keep, but each tank differs anyway so i dont really have anything to suggest.
Imo youll see other signs in the tank (algae, diatoms, etc) that your water is the issue, and it wouldnt only effect the corals.
As for your light, I really cant comment because I have an AIO biocube and the stock lighting has always worked for me. The only coral light related advice i could give is start low. I place a coral on bottom and work it upto the light until its happy. Better to have not enough light and be able to add then to scorch and lower.
When i first started I swear I almost killed a Xenia. I had heard light was really all it needed and it "was unkillable". It started great and for a couple weeks everything seemed fine. After about a month though it started to turn white and started to "melt" (droopy and sad, open but stagnant) and my GSP started to seem frozen, not growing or shrinking. At the time that was all I had cause they were cheap, "indestructible", and I was learning.
When I looked for advise, here, the lfs, FB forums/clubs etc, all i heard was "xenia cant die" , "gsp is pretty hearty itll be fine", "your params seem ok, do you add/dose/test....", "water, RODI, solids, etc" , "add this, change that, spend more $".
Stop! Back to basics.
If tests 1 - 8 were good, logic says the water and equipment wasnt the issue.
So i started surprise nighttime inspections hoping to catch a culprit but after a week or so of no sleep and dragging work shifts, I found a big fat nothing.
This started me thinking a little more logically.
How do coral live normally? What do they eat? How do they catch the food? How do they eat (chew, swallow, absorb)the food?
no coral lives purely by photosynthesis. They do have "mouths" and they do "eat" physical foods (proteins, fat, fiber, etc). Coral ingest food by "breathing" for lack of a better term and the micro particles are trapped in its throat where they are chemically broke down and distributed through the body.
Coral require about 10 - 15 mins of "slack water", no flow, to properly ingest food. So even the lowest flow can keep coral from capturing a "bite". Think trying to eat a slice of pizza thats swinging back and forth on a string. Maybe once and awhile you may get a bite but those few and far between bites wouldnt be enough to sustain longterm.
When testing water, its possible to see the numbers you want in the water but that doesnt mean thats the number the corals are getting. the "swinging pizza" is always hanging there but you still only get an occasional bite.
After reading that i went and bought coral food (kent marine microvert), just the basic sold at the petco down the way. I started target feeding once a day, twice a week with all the powerheads and pumps turned off for 10mins. Literally overnight the 1st night time I noticed a very visible positive reaction. Not long after I found reef roids and now, i cant keep the corals from growing. My clowny almost never leaves his Xenia now.
So to sum up my incredibly and needlessly long story, to me it sounds like theyre simply starving slowly.
* I apologize if feeding was already covered. I might have missed it reading through the all the comments. Its been a long day.